Make navigation set for all versions of all components available via UI model
Antora builds a catalog that contains the navigation set for all components and their versions before converting any of the pages (called the navigation catalog). However, it currently only passes the navigation set for the component version for the current page to the UI model (as page.navigation). Certain UI designs may require access to a broader scope.
Given that Antora doesn't do any processing of the navigation set per page (which is left up to the page template), there's no reason Antora couldn't make the entire navigation set for all components/versions accessible from the UI model.
What seems most logical is to make the navigation set for a given component version available from the data model for that component version (adjacent to the url and version string). This binding to the component version would happen when the navigation set for that component version is built. From the UI model, it would then be accessible via the path `site.components.0.versions.0.navigation or site.components.0.latest.navigation.
Another option would be to expose the navigation catalog to the UI model...but I think runs the risk of leaking too many internals and might not offer the most template-friendly interface.
NOTE: This change would benefit from the change proposed in #253 (closed), as it would make navigating the components easier. For example,